1. Field of the Invention
This invention relates to an enclosed structure and has been devised particularly, though not solely, as a grain silo.
2. Description of the Prior Art
Most silos and containers or storage bins for large amounts of particulate solids, are made from reinforced concrete or heavy steel plate, or from a combination of heavy steel plate and lightweight steelwork. Such constructions require skilled engineering and manpower to erect and are also very costly to build.
Some silos are made entirely of lightweight steelwork but are in practice found to be susceptible to damage by wind when they are empty and to damage by grain pressures when they are being unloaded in a non-uniform way. Some of these structures are fabricated with corrugated sheeting disposed so that the ribs run horizontally that is circumferentially, around the silo. These structures are unhygienic since grain and grain dust tend to lodge on the corrugations and in laps and gaps in the sheeting so that they are prone to insect infestation.
It has been found in practice a desirable objective to dispose lightweight corrugated wall sheeting of grain silos in such a way that the ribs run vertically rather than circumferentially. This provides greater hygiene for storage of grain, since grain will then not readily adhere to wall. However it has been found in practice there are difficulties in this vertical arrangement of the wall sheeting corrugations.
Firstly there is a difficulty due to stiffness in the vertical direction which is caused by the phenomenon of the walls expanding outwards elastically as the silo is filled. In practice, this expansion causes a problem where the cladding is fastened to the floor or foundation which is unyielding. The vertical stiffness of the cladding material makes it unable to accommodate this movement above and the restraint below and this may lead to the sheeting being sheared open or cracked after a number of loadings.
There is also serious difficulty due to lack of stiffness in the horizontal direction which results in a weakness in resisting wind loads. So, in practice, wind girders have to be provided to safeguard the silo when it is empty.
A solution to the first of these difficulties has been proposed in U.S. Pat. No. 4,453,351 according to which a heavy steel plate is utilised for the bottom section of the wall. The plate is fixed the silo foundatios along its lower edge and to the cladding along its upper edge and has sufficient strength and flexibility to accomodate expansion of the silo. The steel plate arrangement is however, heavy and accordingly adds to the difficulty of assembly and cost of the silo.
U.S. Pat. No. 4,453,351 also describes a girt arrangement for stiffening the silo in which the girts comprise flat steel members joined by bolted steel angles welded to the girts. This arrangement can withstand the pressure of grain loads and the eccentric pressures due to wind loads and uneven grain loads, however the girts are heavy and significantly add to the cost of the silo. In addition, the flat steel girts do not provide significant strength against inwardly directed wind forces when the silo is empty so that additional strengthening is required to obtain sufficient resistance to wind damage.
The girts disclosed in U.S. Pat. No. 4,453,351 were joined using heavy steel plates and angle bars welded to the outside of the girts. This was due to the need to maintain a flush inside face in the girt members, because of the presence of the adjacent wall cladding, which inhibits the use of bolts passing through the girts. The connecting pieces in that invention were found in practice to be necessarily heavy and added significantly to the cost of the structure.